Selective catalytic reduction on n2o

ABSTRACT

A gas stream containing nitrous oxide and ammonia is contacted with a catalyst composition containing a zeolite. N 2 O is reduced to N 2  and H 2 O at low temperatures in a highly efficient manner. Ammonia-mediated reduction of nitrous oxide can be effectuated from gas streams having N 2 O concentrations as low as 1%. The gas stream may also contact a catalytic composition selective for the reduction of NO x . In this way, N 2 O and NO x  treatment may be effectuated in a single process stream.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an improved method for the reductionof nitrous oxide; and more particularly, to ammonia-mediated reductionof nitrous oxide.

2. Description of the Prior Art

Nitrous oxide (N₂O) is not commonly considered an atmospheric pollutantand has not been considered a constituent of the gaseous pollutantscollectively referred to as nitrogen oxides (NO_(x)) which have receivedwide attention as pollutants harmful to the b environment. However,recent studies indicate that N₂O in the Earth's atmosphere may beincreasing at a rate of about 0.2% per year and that this increaseappears to be caused by anthropogenic activity.

N₂O is a major stratospheric source of NO, is believed to be involved indestroying the ozone layer and is recognized to be a greenhouse gas.Because N₂O has an atmospheric lifetime of approximately 150 years,researchers are attempting to identify sources of the pollutant and tolimit further production of the harmful gas. Recent reports such as anarticle by Thiemens and Trogler, Science, 251 (1991)932 suggest thatvarious industrial processes significantly contribute to the increasedlevels of N₂O found in the Earth's atmosphere.

For example, nitrous oxide is a by-product formed during the manufactureof monomers used in producing 6,6- and 6,12-nylon. Nylon polymers aretypically formed by subjecting a dicarboxylic acid and a diamine to acondensation polymerization reaction. The most widely used dicarboxylicacid, adipic acid, is prepared primarily by oxidizing cyclohexane in airto form a cyclohexanol/cyclohexanone mixture followed by oxidizing suchmixture with HNO₃ to form adipic acid and N₂O. Thiemens and Troglercalculate that about 1 mol of N₂O per mole of adipic acid is formed as aside product in adipic acid processes. Assuming that 2.2×10⁹ kg ofadipic acid are produced globally per year, about 1.5×10¹⁰ mol yr⁻¹ ofN₂O by-product or 10% of the annual output of atmospheric N₂O can beattributed to this single process. Also, for many industrial processes,N₂O may be co-present with nitrogen oxides, NO_(x) (NO and NO₂), in theeffluent gases.

M. Schiavello and coworkers, (J. Chem. Soc. Faraday Trans. 1, 71(8),1642-8) studied various magnesium oxide-iron oxides and magnesiumoxide-iron oxide-lithium oxide systems as N₂O decomposition catalysts.While magnesium oxide-iron oxide samples which were fired in air andwhich contained Mge₂O₄ demonstrated low activity, similar samples firedunder reducing atmospheres and containing Fe²⁺ in solid solutiondemonstrated greater activity. The researchers concluded that Fe³⁺ ionsin the ferrite phase are not catalytically active toward the subjectreaction whereas Fe³⁺ ions contained in MgO together with Li⁺ arecatalytically active when the ratio of lithium to iron is less than 1.

P. Porta and coworkers (J. Chem. Soc. Faraday Trans 1, 74(7), 1595-603)studied the structure and catalytic activity of Co_(x)Mg_(1-x)Al₂O₄spinel solid solutions for use as catalysts in decomposing N₂O intogaseous nitrogen and oxygen. The catalytic activity per cobalt ion invarious N₂O decomposition catalysts was found to increase withincreasing dilution in MgO. The distribution of cobalt ions amongoctahedral and tetrahedral sites in the spinel structure ofCo_(x)Mg_(1-x)Al₂O₄ was found to vary with temperature and the fractionof cobalt ions in octahedral sites was found to increase with increasingquenching temperature. The researchers concluded that catalytic activitygenerally increases as a greater amount of cobalt ions is incorporatedinto octahedral sites in the structure.

W. Reichle (Journal of Catalysis 94 (1985) 547) reported that variousanionic clay minerals belonging to the pyroaurite-sjogrenite group, suchas hydrotalcite (Mg₆Al₂(OH)₁₆(CO₃ ²⁻).4H₂O can be thermally decomposedto form a product which is a useful catalyst for vapor-phase aldolcondensations. Replacement of Mg by Fe, Co, Ni and Zn and/or replacementof Al by Fe and Cr also results in isomorphous double hydroxides which,on heat treatment, are rendered catalytically active. The reference alsostates that the activity of the catalyst is strongly affected by thetemperature at which the hydrotalcite is activated.

Commonly owned U.S. Pat. No. 5,171,553 discloses a highly efficient,commercially viable process for removing N₂O from gaseous mixtures. Theprocess utilizes catalysts comprising a crystalline zeolite which, atleast in part, comprise five membered rings having a structure typeselected from the group consisting of BETA, MOR, MUI, MEL and FERwherein the crystalline zeolite has been at least partiallyion-exchanged with a metal selected from the group consisting of copper,cobalt, rhodium, iridium, ruthenium and palladium.

Likewise, commonly owned U.S. Pat. No. 5,407,652 discloses an efficientcatalytic pollution control process for removing N₂O from gaseousmixtures. The process utilizes catalysts derived from anionic clayminerals such as hydrocalcites, sjogrenites and pyroaurites which, afterappropriate heat activation, provide superior N₂O decompositionactivity.

While the prior art has shown an awareness of the decomposition of N₂Ointo its respective components, industry urgently needs to developenhanced catalytic processes for destroying N₂O emissions prior to theventing of commercial process effluent streams into the atmosphere. Thisneed is particularly critical with respect to effluent streamscontaining low levels of this contaminant. In addition, methods areneeded to remove this contaminant from engine exhaust streams. It wouldbe particularly useful if the catalytic decomposition of N₂O could becombined with reduction of NO, so as to economically and efficientlyremove these pollutants from both industrial effluent streams and engineexhaust streams.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forammonia-mediated reduction of nitrous oxide comprising contacting a gasstream containing nitrous oxide and ammonia with a catalyst compositioncomprising a zeolite. Advantageously, N₂O is reduced to N₂ and H₂O atlower temperatures and with greater efficiency than heretofore known inthe art. In this way, the present invention provides an economical andreliable control method for nitrous oxide pollution.

In another aspect of the invention there is provided a method forammonia-mediated N₂O and NO, reduction comprising contacting a gasstream containing ammonia with a catalyst composition containing anupstream catalyst and a downstream catalyst as sensed relative to thesequence of flow of the gaseous stream through the catalyst wherein theupstream catalyst is selective for the reduction of NO, and thedownstream catalyst is selective for the reduction of N₂O.Alternatively, this catalyst configuration may be reversed.Advantageously, the upstream and downstream catalysts can comprise thesame material. The ability to control N₂O and NO, in a single processstream and, where desired, with a single catalytic material, results insignificant cost savings. Such a combined process is particularly usefulin industries and in engine exhaust streams where N₂O and NO_(x) arepresent in the outgas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is had to the following detaileddescription and the accompanying drawings, in which:

FIG. 1 is a graph of laboratory data showing % N₂O conversion versuscatalyst temperature for a zeolite based Fe-exchanged catalyst;

FIG. 2 is a graph depicting % ammonia conversion versus catalysttemperature for the same test series as in FIG. 1;

FIG. 3 is a graph depicting % N₂O conversion versus catalyst temperaturefor a V₂O₅/TiO₂ catalyst;

FIG. 4 is a graph depicting % ammonia conversion versus catalysttemperature for a V₂O₅/TiO₂ catalyst;

FIG. 5 is a graph depicting % N₂O conversion versus catalyst temperaturefor a Pt/Au catalyst;

FIG. 6 is a graph depicting % ammonia conversion versus catalysttemperature for a Pt/Au catalyst;

FIG. 7 is a graph of laboratory results showing the use of a zeolitecatalyst to remove NO_(x) and N₂O in the absence of NH3.

FIG. 8 is a graph of laboratory results showing the use of a zeolitecatalyst to achieve reduction for both NO_(x) and N₂O gases byintroducing NH₃ into the gas stream;

FIG. 9 is a schematic of the apparatus for the control of N₂O gas; and

FIG. 10 is a schematic of the apparatus for the control of N₂O andNO_(x).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a highly efficient catalytic method forconverting nitrous oxide (N₂O) into environmentally safe products,namely gaseous nitrogen and water. The method is based on the surprisingdiscovery that adding ammonia to a gas stream containing N₂O and passingthe mixture over a zeolite catalyst composition results in enhanced N₂Oreduction than otherwise known in the art. Specifically, the N₂Odecomposition rate is enhanced when ammonia is introduced to the gasstream. In so doing, the introduced ammonia is also converted into N₂and water, with its own conversion rate increased by the presence ofN₂O. As a result of this enhanced N₂O decomposition rate, the method iscapable of reducing N₂O at much lower temperatures and/or lower catalystvolumes than presently required. Moreover, the enhanced N₂O removaloffered by the present invention permits the removal of low levels ofN₂O (i.e., less than 1% N₂O) from process streams. The method can alsobe combined with selective catalytic reduction (SCR) of nitrous oxides(NO_(x)) to achieve simultaneous removal of N₂O and NO_(x). This isparticularly advantageous for internal combustion engines or industrialprocesses where N₂O and NO, are both present in the outgas.

In accordance with the present invention, there is provided a method forthe reduction of a gas stream containing nitrous oxide which comprisescontacting a gas stream containing N₂O with ammonia over a catalystcomposition comprising a zeolite in order to catalyze the reduction ofthe N₂O with ammonia. Such gaseous streams, for example, the products ofcombustion of internal combustion engines, boilers, and the nitric acidmanufacturing process often inherently contain substantial amounts ofoxygen. These exhaust gases contain from about 2 to 15 volume percentoxygen and from about 20 to 100,000 volume parts per million (ppm) N₂O.Zeolites, and in particular, metal-promoted zeolites can be used topromote the reaction of ammonia with N₂O to form nitrogen and H₂Oselectively over a competing reaction of oxygen and ammonia.

It is desirable in the method to provide sufficient ammonia to reactcompletely with the N₂O present in order to drive the reaction tocompletion. However, in practice, significant excess ammonia is normallynot provided because the discharge of unreacted ammonia from thecatalyst to the atmosphere would itself engender an air pollutionproblem. Accordingly, the ratio of ammonia to N₂O in the gas streamshould range up to about 2.0 ppm NH₃/ppm N₂O based on the total volumeof the gas stream in order to impact N₂O reduction. The NH₃/N₂O ratioshould be at least 0.5, with the most preferred ratio being from about0.8 to about 1.0 NH₃/N₂O. Addition of the foregoing amounts of ammoniato the gas stream advantageously enhances N₂O conversion over processstreams lacking ammonia.

As previously indicated, the method of the present invention is usefulfor ammonia-mediated N₂O reduction at lower processing temperatures thanpreviously known in the art. In accordance with the present invention,N₂O reduction can be enhanced over conventional processes attemperatures of greater than 250° C. and preferably those ranging from350° C. to 600° C. and most preferably 450° C. to 600° C.

Generally, any suitable zeolitic material may be utilized in thecatalyst compositions of the invention. Preferably, the zeoliticmaterials are ion-exchanged with Fe, Cu, Co, Ce, Pt, Rh, Pd, Tr, Mg.Ion-exchanging the zeolitic materials serves to enhance the catalyticactivity toward and selectivity for the reaction between NH₃ and N₂O.Useful zeolites for practice of the present invention includecrystalline zeolites which, at least in part, comprise five memberedrings having a structure type selected from the group consisting ofBETA, MOR, MFI, ZSM, MEL, FER and Y. Of these, BETA, ZSM, MOR and Y areparticularly preferred, and BETA is the most preferred. Suitable ionexchange compounds include Fe, Cu, Co, Ce, Pt, Rh, Pd, Ir, and Mg, withFe, Cu, Co, Ce, Pd, Rh and Fe, Ce, Cu, Co and combinations thereof mostpreferred. Such ion-exchange techniques are well known in the art andare reviewed in Breck, Zeolite Molecular Sieves, Structure, Chemistryand Use, Chapter 7, Ion-Exchange Reactions and Zeolites, beginning onpage 529, published by John Wiley and Sons, New York, 1974 which isexpressly incorporated herein by reference.

Because the invention is capable of reducing N₂O at lower temperatures,removal efficiencies of >90% can be attained at temperatures of 300° C.to 600° C., with removal efficiency calculated as [[(inlet N₂O(moles)-outlet N₂O (moles))/inlet N₂O moles]* 100]. In addition, themethod of the present invention can also remove small quantities of N₂Ofrom gas streams having low levels of this contaminant; for example,less than 1%. Advantageously, the method of the present invention canremove N₂O from gas streams containing as much as about 5000 ppm N₂O toas low as 20 ppm at these temperatures.

The method described herein may also be combined with selectivecatalytic reduction of NO_(x) to achieve simultaneous removal of NO_(x)and N₂O from a single process stream. For example, many industrialprocess streams and engine exhausts contain NO_(x) and N₂O. Prior to thepresent invention, there was no suitable method for controlling both ofthese contaminants in a single process stream, due to the fact that mostSCR catalysts are operated at less than 550° C. and no suitable catalystwas found to destroy N₂O effectively at those temperatures.

Where it is desired to simultaneously remove NO_(x) and N₂O from asingle process stream, ammonia is introduced into the process streamupstream of the catalyst bed. The catalyst bed contains an upstreamcatalyst and a downstream catalyst. Three arrangements can be made toachieve the simultaneous removal. In one arrangement, the upstream anddownstream catalysts comprise the same material which can promote boththe selective reduction reaction of NO_(x) and reduction of N₂O. Forthis arrangement, a zeolite-based SCR catalyst, such as Fe/Beta, can bevery effective. The ability to control NO_(x) and N₂O in the samereactor will result in significant savings in control costs. For thisarrangement, the exhaust temperature is preferably in the range of 350°C. to 600° C. The second arrangement comprises an upstream catalystwhich is selective for the reduction of N₂O and a downstream catalystselective for the reduction of NO_(x). In this arrangement, the ammoniarequired for N₂O and NO_(x) removal is introduced before the upstreamN₂O catalyst, which gives higher NH₃ concentration to further enhancethe N₂O removal rate. The unconverted ammonia is then reacted withNO_(x) over the downstream SCR catalyst and converted to N₂ and water.This arrangement is advantageous for streams that contain a highconcentration of N₂O relative to NO_(x), since the downstream SCR bed isused to convert the low level of NO_(x) and NH₃. The third arrangementcomprises an upstream catalyst which is selective for the reduction ofNO_(x) (SCR NO_(x)) and a downstream catalyst which is selective forreduction of N₂O. This arrangement is advantageous for treating streamsthat contain high concentrations of NO, relative to N₂O. The unconvertedammonia coming off of the catalyst selective for the reduction of NO_(x)is used to promote reduction of N₂O over the downstream catalyst.Methods and suitable catalytic materials for removing NO_(x) are wellknown in the art and are described in detail in commonly owned U.S. Pat.No. 5,024,981 of Barry K. Speronello et al., entitled “StagedMetal-Promoted Zeolite Catalysts and Method for Catalytic Reduction ofNitrogen Oxides Using Same,” and commonly owned U.S. Pat. No. 4,961,917of John W. Byrne entitled “Zeolite Catalysts and Method for Reduction ofNitrogen Oxides With Ammonia Using Same,” both of which are expresslyincorporated herein by reference.

In this aspect of the invention, the catalysts are arranged in at leasttwo zones in which one zone contains a catalyst selective for thereduction of NO_(x), and the other zone contains a zeolite catalystselective for the reduction of N₂O. Any suitable form of the catalystmay be used in these and other aspects of the invention, such as amonolithic honeycomb-type body containing a plurality of fine parallelgas flow passages extending therethrough, the walls of which are coatedwith the catalytic material. Typically, such monolithic bodies are madeof a refractory ceramic material such as cordierite, mullite oraluminia, and the catalytic material coating the fine gas flow passagesis contacted by the gaseous stream as it flows through the gas flowpassages. Separate monolith bodies may be used for each of the zones. Asindicated above, each of the zones preferably comprises the samecatalytic material.

The catalyst may also take the form of a packed bed of pellets, tablets,extrudates or other particles of shaped pieces, such as plates, saddles,tubes or the like. The physical configuration of the catalyst used in agiven case will depend on a number of factors such as the spaceavailable for the catalytic reactor, the activity of the catalyticmaterial utilized, and the permitted or desired amount of pressure dropacross the catalyst bed; for example, where the method is used to treatengine exhausts. A preferred physical configuration of the catalyst isone which provides parallel flow passageways for the gas, such as thosefound in the above-described honeycomb-type catalysts. Otherarrangements providing such parallel flow passageways include the use ofparallel plates or stacked tubes. Because of its ease of handling andinstallation as well as good mass transfer characteristics relative toother parallel passage configurations, a highly preferred physicalconfiguration of the catalysts of the invention is a monolithichoneycomb member having relatively high cell (flow passageway) densityof approximately 60 cells or more per square inch of end face of thehoneycomb member. The walls defining the gas flow passages (or cells)are desirably as thin as possible consistent with the requisitemechanical strength of the honeycomb. Catalysts used in the inventionmay take the form of a monolithic honeycomb carrier, the gas flowpassages of which comprise or are coated with separate zeolite catalyticcompositions as described above. For example, a catalytically inerthoneycomb member, such as a cordierite carrier, may be coated with awashcoat of fine particles of a catalyst selective for the reduction ofN₂O. Alternatively, a powder of a catalyst selective for the reductionof N₂O may be mixed with a binder and extruded into the honeycombconfiguration. In another approach, the catalytic material may be formedin situ by preparing the honeycomb structure from a zeolitic precursorraw material which is then treated to form the zeolitic material as partof the honeycomb structure.

The following examples are presented to provide a more completeunderstanding of the invention. The specific techniques, conditions,materials, proportions and reported data set forth to illustrate theprinciples and practice of the invention are exemplary and should not beconstrued as limiting the scope of the invention.

EXAMPLE 1 Synthesis of N₂O Reduction Catalysts

A zeolite catalyst was prepared according to the following generalprocedure. Zeolite Beta powder was prepared via the synthetic proceduresdisclosed in Example 1 (Batch 1) of commonly owned U.S. Pat. No.4,961,917 of John W. Byrne, which is expressly incorporated by referenceherein. The powder was then ion-exchanged with Fe by dispersing 212.5 gof zeolite Beta powder in a preheated, 70° C. solution consisting of1000 g of deionized water and 25.5 g of iron sulfate heptahydrate. Thepreparation was mixed for 1 hour, after which the zeolite powder wasfiltered from the solution followed by water washing to remove residualsulfate. The filtered cake was then mixed with deionized water in theproportion 40% zeolite/60% water by weight, and the mixture was placedin a high shear mixer to form a washcoat slurry containing zeoliteFe/Beta with a particle size 90% less than 20 μm. A monolith support ofcordierite containing 100 cells per square inch of cross section wasdipped into the washcoat slurry. After calcination at 400° C., thesupport contained 1.5 g of zeolite re/Beta/in³.

EXAMPLE 2 Catalytic Decomposition of N₂O

The following general procedure was utilized for catalyticallyconverting N₂O with ammonia to gaseous nitrogen and water A core sampleof Fe/Beta on 100 CPSI honeycomb was loaded into a 1 inch reactor. A gasstream consisting of varied concentrations of N₂O and ammonia, 10% O₂,10% 1120, and balanced with N₂ was fed through the catalyst at a flowrate equivalent to 20,000 hour⁻¹ space velocity (“SV”), which is definedas [(gas flow rate at 25° C. (liters/hr))/(catalyst volume (liters))].The disappearance of both N₂O and ammonia across the catalyst weremeasured by taking gas samples before and after passing over thecatalyst. The gas samples were then measured by on-line N₂O and ammoniainfrared analyzers, such as Sieman N₂O (Ultramat 5E) and NH3 (Ultramat5F) analyzers. These conversions were then measured over a temperaturerange of 250° C. to 450° C. The tests were conducted with varyingconcentrations of N₂O, and ammonia: (a) 200 ppm N₂O, 0 ppm ammonia; (b)200 ppm N₂O, 200 ppm ammonia; and (c) 0 ppm N₂O, 200 ppm ammonia. Asillustrated in FIG. 1, N₂O conversion was low (i.e., 10%) attemperatures ranging from 250° C. to 450° C. in the absence of ammonia.With 200 ppm ammonia present, N₂O conversion increased substantiallywith increasing temperature. By adding ammonia, N₂O conversion increasedsignificantly at temperatures greater than 300° C. These datademonstrate that the presence of ammonia substantially increased thereduction rate of N₂O, even at low temperatures (i.e., above 250° C.).

FIG. 2 illustrates the disappearance of ammonia. In the absence of N₂O,ammonia conversion was very low, indicating that very little ammonia wasoxidized. However, in the presence of N₂O, the ammonia disappearancerate increased substantially. Thus, it is apparent that ammonia and N₂Omutually enhance the conversion of each other to N₂ and H₂O over azeolite-based catalyst.

EXAMPLE 3 Effect of a Zeolite Catalyst on Ammonia-Mediated N₂O Reduction

In order to ascertain if ammonia-mediated N₂O reduction is unique tozeolite catalysts, the procedures described in Example 2 were conductedutilizing two other catalytic compositions, V₂O₅/TiO₂, and Pt/Au. Thesecompositions were obtained as follows:

V₂O₅/TiO₂

130.8 g of citric acid was mixed with 1000 g of deionized water, and themixture was heated to 80° C. to dissolve the citric acid. This solutionwas then combined in a mixing tank with 37.5 g of ammonium metavanadate,followed by an additional 700 g of deionized water. 1425 g of TiO₂powder having a BET surface area of 100 m²/g was added to the solutionto obtain a 2% V₂O₅/TiO₂ washcoat slurry. A monolith support ofcordierite containing 100 cells per square inch of cross section wasdipped into the washcoat slurry. After calcination at 400° C., thesupport contained 1.5 g of V₂O₅/TiO₂ catalyst powder/in³.

Pt/Au

177 g of gamma alumina powder having a BET surface area of 150 m²/g wasball milled with deionized water and acetic acid to form a 50% solidslurry. The slurry was then placed in a dispersion tank and combinedwith 0.83 g of Pt equivalent amine-solubilized aqueous platinumhydroxide (H₂Pt(OH)₆) solution and 0.17 g of Au equivalent aqueous(HAuCl₄.3H₂O) solution. A monolith support of cordierite containing 100cells per square inch of cross section was dipped into the washcoatslurry. After calcination at 400° C., the support contained 1.7 g ofAl₂O₃/in³, 40 g of Pt/ft³, and 8 g of Au/ft³.

FIG. 3 illustrates that for the V₂O₅/TiO₂ catalyst, there was verylittle N₂O reduction over the tested temperature range, with or withoutthe presence of ammonia. Unlike the zeolite catalyst, the V₂O₅/TiO₂catalyst did not show any activity for N₂O. FIG. 4 demonstrates thatammonia conversion is not affected by the presence of N₂O. In contrastto the zeolite catalyst, the reduction rate of N₂O over the V₂O₅/TiO₂catalyst could not be promoted by injecting ammonia into the gas stream.These results demonstrate that the V₂O₅/TiO₂ catalyst does little tofoster the interaction between N₂O and ammonia. Also, even though theV₂O₅/TiO₂ catalyst converted some ammonia at temperatures in the 350° C.to 450° C. range, the conversion was not increased when N₂O was presentin the gas stream. These results show that the mutually enhancedconversion of NH₃ and N₂O to N₂ and H₂O is specific to the uniquecatalytic properties of the zeolite catalyst. It is clear that thecombination of ammonia addition and the use of a zeolite-based catalystis essential to enhance the N₂O removal rate.

FIGS. 5 and 6 are graphs on N₂O and ammonia conversions, respectively,for a Pt/Au catalyst. As shown in these figures, the Pt based catalystwas not active to decompose N₂O, but was very active to oxidize ammonia.When ammonia and N₂O are co-present in an oxidizing environment, the N₂Oconversion could become negative, indicating that some N₂O was formedthrough the ammonia oxidation reaction. Thus, for this catalyst, thepresence of ammonia not only fails to enhance N₂O conversion, but alsonegatively affects N₂O removal efficiency.

FIGS. 7 and 8 are graphs of laboratory results showing the use ofzeolite catalysts to achieve reduction for both NO_(x) and N₂O gases byintroducing ammonia into the gas stream. For this test, the inlet gascontained 815 ppm N₂O and 52 ppm NO_(x). The conversions of N₂O andNO_(x) across a Fe/Beta catalyst were measured at 450° C. and 500° C.FIG. 7 shows that with no ammonia, there was no NO_(x) conversion, andthe N₂O conversions were 30% and 78%, respectively, at 450° C. and 500°C. FIG. 8 shows that by introducing 811 ppm NH₃ to the gas stream, bothNO_(x) and N₂O conversions were increased substantially. For NO_(x)removal, greater than 98% conversion was achieved at both temperatures.For N₂O removal, conversion was increased to 80% at 450° C. and 99% at500° C. These results demonstrate that the zeolite catalyst is veryactive to promote the selective catalytic reduction of NO_(x) withammonia to form N₂ and water. Additionally, the combination of NH₃ andthis zeolite catalyst substantially improved the N₂O conversionefficiency at the lower temperature.

FIG. 9 is a schematic of the apparatus for the control of N₂O gas. Inthis schematic, ammonia is introduced to a gaseous stream containing N₂Oat a ratio of about 1:1. This stream is then passed through azeolite-based catalyst, which promotes the mutually enhanced removalrates of N₂O and NH₃.

FIG. 10 is a schematic of the apparatus for the simultaneous control ofN₂O and NO_(x). In this schematic, the gaseous stream is introduced withsufficient quantity of ammonia. This stream is then passed through azeolite-based catalyst which promotes the mutually enhanced rates of N₂Oand ammonia removal, as well as the selective catalytic reduction ofNO_(x).

Having thus described the invention in detail, it will be recognizedthat such detail need not be strictly adhered to but that variouschanges and modifications may suggest themselves to one skilled in theart, all falling within the scope of the invention, as defined by thesubjoined claims.

1.-16. (canceled)
 17. A catalyst composition for ammonia-mediatedremoval of N₂O from a gas stream comprising a catalyst selective for thereduction of N₂O. 18.-21. (canceled)
 22. A catalyst composition forammonia-mediated N₂O and NO_(x) reduction comprising at least two zones,at least one of the zones comprising a catalyst material selective forthe reduction of NO_(x) and at least one other zone comprising acatalyst material selective for the reduction of N₂O.
 23. A catalystcomposition as recited in claim 22, wherein at least two zones have thesame catalyst composition.
 24. A catalyst composition as recited inclaim 22, wherein the catalyst composition has two zones, each havingthe same catalyst composition.